The efforts of the MMTP Section are directed at understanding the early events in the interaction of phorbol ester tumor promoters with cells and tissues. In addition to the previously demonstrated membrane receptors for the phorbol esters, specific phorbol ester binding activity has been characterized in cytosol in the form of a phorbol ester apo-receptor which has an absolute phospholipid requirement. Different phospholipids vary in their ability to reconstitute binding activity. Phosphatidylserine is most effective and phosphatidylcholine is least effective. The structure-activity requirements for binding by the reconstituted apo-receptor closely resemble those for the membrane receptor, suggesting they may be different forms of the same receptor. The apo-receptor co-fractionates with protein kinase C, thus identifying a biochemical function for the receptor. Diacylglycerol, an activator of protein kinase C, competitively inhibits phorbol ester binding, consistent with its being an (the) endogenous phorbol ester analog. The 50% inhibitory concentration for the diacylglycerol diolein is 0.4% of the concentration of phospholipid. Evaluation of different diacylglycerol analogs indicates that short or unsaturated side chains are required for activity. The ability of diacylglycerols to act as phorbol ester analogs suggests that the receptor recognizes that fraction of the phorbol esters which is dissolved in the lipid phase rather than that fraction in aqueous solution. Results with the highly lipophilic phorbol 12,13-didodecanoate confirm this prediction. Techniques for isolation of the receptor in good yield from membranes and cytosol are being developed. The role of the lipid environment in binding and response is being characterized, both by reconstitution and by photoaffinity labeling. Multiple phorbol ester receptors have been implicated in the heterogeneity of phorbol ester responses. The binding characteristics of intact, cultured keratinocytes change from homogeneous to heterogeneous as differentiation proceeds.